Bias-instability reduction in fiber optic gyroscopes

ABSTRACT

Apparatus for providing Bias-Instability reduction in Fiber Optic Gyroscopes are provided. In one embodiment, an optical circuit for a fiber optic gyroscope having a broadband light source and an optical fiber loop comprises: a PM fiber of length v; an IOC coupled to the PM fiber via a pigtail of length d 1 , a second pigtail with length of d 2 , and a third pigtail with length of d 3 ; a splitter that splits light received from the first pigtail into a first and second beams directed to the second and third pigtails; and a depolarizer circuit coupled to said fiber loop including a first fiber section of length x, a second fiber section of length z, a third fiber section of length w and a fourth fiber section of length y; and wherein the lengths of v, x, y, d 1  and d 3  are proportioned to avoid regions of high bias instability.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,U.S. patent application Ser. No. 12/187,932 (hereafter “the '932Application”), entitled “BIAS-INSTABILITY REDUCTION IN FIBER OPTICGYROSCOPES” filed on Aug. 7, 2008.

GOVERNMENT LICENSE RIGHTS

The U.S. Government may have certain rights in the present invention asprovided for by the terms of a Government Contract No. N00030-08-C-0010with the U.S. Navy.

BACKGROUND OF THE INVENTION

In a fiber optic gyroscope, polarization errors result from interferencebetween the primary lightwaves and spurious lightwaves, as well asinterference among spurious lightwaves. The weak spurious lightwavescome from polarization cross-couplings at non-ideal fiber splices, inbirefringent coil fiber, at junctions between integrated optical circuit(IOC) waveguides and its pigtail fibers, and inside optical components.These spurious lightwaves reaching the photodetector carry erroneousSagnac phase information because they traveled nonreciprocal paths inthe sensing loop. Methods of using Lyot-type fiber depolarizers andtailoring of the depolarizer polarization maintaining (PM) fiber lengthshave been suggested to mitigate polarization errors. In prior artpolarization error models, the polarization errors are evaluated in thetime domain by keeping track of time and phase delays of spuriouslightwaves originating from cross-couplings in the optical circuit.Wavelength dependent properties of the optical component, such aspolarization dependent loss (PDL), polarization mode dispersion (PMD),etc., are often completely or partially ignored in the model.Specifically, for a depolarized gyroscope using a non-polarizationmaintaining single-mode (SM) fiber coil, the impact of the SM coilbirefringence on the polarization error is empirically taken intoaccount by assuming a broadened light source coherence function. Suchsimplifications lead to inaccuracies (up to one order of magnitude ofdeviation) in evaluation of polarization errors. More accurate modelingmethods are needed to find optimal design parameters of interferometricfiber optic gyroscopes with reduced polarization error and biasinstability.

SUMMARY

In one embodiment, an optical circuit for a fiber optic gyroscope havinga broadband light source and a single mode optical fiber loop comprises:a polarization maintaining (PM) delay management fiber of length v,having a first end and a second end, wherein the first end of thepolarization maintaining delay management fiber is coupled a directionalcoupler that receives light from the broadband light source; anintegrated optical circuit (IOC) having the integrated optical circuitcoupled to the second end of the polarization maintaining delaymanagement fiber via a first polarization maintaining fiber pigtail oflength d₁; the integrated optical circuit coupled to a secondpolarization maintaining fiber pigtail with length of d₂; and theintegrated optical circuit coupled to a third polarization maintainingfiber pigtail with length of d₃; wherein the integrated optic circuit(IOC) comprises a polarization maintaining waveguide device havingwaveguide length of L_(IOC) and comprise a polarizing element so thatthe total effective polarization extinction ratio (PER) of theintegrated optic circuit is ε. The circuit further comprises a splitterthat splits light received from the first polarization maintaining fiberpigtail into a first beam directed to the second polarizationmaintaining fiber pigtail and a second beam directed to the thirdpolarization maintaining fiber pigtail; and a depolarizer circuitcoupled to said fiber loop, said depolarizer region including a firstoptical fiber section of length x, a second optical fiber section oflength z, a third optical fiber section of length w and a fourth opticalfiber section of length y, each comprising polarization maintainingfibers having a beat length of L_(B); wherein the second optical fibersection is coupled to the first optical fiber section via a firstsplice, the first optical fiber section is coupled to a first end of thesingle mode optical fiber loop via a second splice, the fourth opticalfiber section is coupled to the third optical fiber section via a thirdsplice, and the third optical fiber section is coupled to a second endof the single mode optical fiber loop via a fourth splice; and whereinx+z is substantially equal to w+y; wherein the lengths of v, x, y, d₁and d₃ are proportioned to avoid regions of high bias instabilitydefined by the expressions v=x−(L_(IOC)+y+d₁+d₃),v=2x−(L_(IOC)+y+d₁+d₃), v=3x−(L_(IOC)+y+d₁+d₃), andv=4x+y−(L_(IOC)+y+d₁+d₃).

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 illustrates an optical circuit of a depolarized gyroscopeaccording to an embodiment of the invention;

FIG. 2 illustrates an example of a bias versus temperature plot for agyroscope with polarization errors obtained according to an embodimentof the invention;

FIGS. 3 a and 3 b illustrate two coil coherence functions and theirassociated wavelength dependent differential group delays (DGDs) inaccordance with an embodiment of the present invention;

FIG. 4 illustrates a comparison of bias thermal instabilities undertemperature ramping for two gyroscopes having identical opticalcomponents except for the coil (coil-I and II, respectively) accordingto an embodiment of the present invention;

FIG. 5 illustrates an example of calculation of relative gyroscope biasfluctuation amplitude as a function of the 2×2 coupler polarizationdependent splitting ratio (PDSR);

FIG. 6 illustrates an example of calculation of relative gyroscope biasfluctuation amplitude as a function of the cross-coupling at splice 32of FIG. 1;

FIG. 7 illustrates an example of calculation of polarization errorinduced relative bias fluctuation amplitude as a function of IOCpolarization extinction ratio;

FIG. 8 illustrates dependence of relative bias fluctuation amplitude onthe length v, of PM fiber 15 of FIG. 1 for a depolarizer designaccording to an embodiment;

FIGS. 9 a and 9 b illustrate a first example of contour plots ofrelative bias fluctuation amplitude as a function of PM fiber lengths, xand v;

FIGS. 10 a and 10 b illustrate a second example of contour plots ofrelative bias fluctuation amplitude as a function of fiber lengths, xand w; and

FIG. 11 illustrates an operating environment in which an embodiment ofthe invention can be implemented.

DETAILED DESCRIPTION

FIG. 11 illustrates an example of a suitable computing systemenvironment 100 on which an embodiment of the invention may beimplemented. The computing system environment 100 is only one example ofa suitable computing environment and is not intended to suggest anylimitation as to the scope of use or functionality of embodiments of theinvention. Neither should the computing environment 100 be interpretedas having any dependency or requirement relating to any one orcombination of components illustrated in the exemplary operatingenvironment 100.

Embodiments of the invention are operational with numerous othergeneral-purpose or special-purpose computing-system environments orconfigurations. Examples of well-known computing systems, environments,and/or configurations that may be suitable for use with embodiments ofthe invention include, but are not limited to, personal computers,server computers, hand-held or laptop devices, multiprocessor systems,microprocessor-based systems, set-top boxes, programmable consumerelectronics, network PCs, minicomputers, mainframe computers,distributed-computing environments that include any of the above systemsor devices, and the like.

Embodiments of the invention may be described in the general context ofcomputer-executable instructions, such as program modules, executed by acomputer. Generally, program modules include routines, programs,objects, components, data structures, and the like that performparticular tasks or implement particular abstract data types.Embodiments of the invention may also be practiced indistributed-computing environments where tasks are performed by remoteprocessing devices that are linked through a communications network. Ina distributed-computing environment, program modules may be located inboth local- and remote-computer storage media including memory storagedevices.

The operating environment illustrated in FIG. 11 typically includes atleast some form of computer readable media. Computer readable media canbe any available media that can be accessed by one or more components ofsuch operating environment. By way of example, and not limitation,computer readable media may comprise computer storage media andcommunication media. Computer storage media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Computerstorage media includes, but is not limited to, RAM, ROM, EEPROM, flashmemory or other memory technology, CD-ROM, digital versatile disks (DVD)or other optical storage, magnetic cassettes, magnetic tape, magneticdisk storage or other magnetic storage devices, or any other mediumwhich can be used to store the desired information and which can beaccessed by one or more components of such operating environment.Communication media typically embody computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism and includesany information delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, RF, infrared and other wireless media. Combinations of the anyof the above should also be included within the scope of computerreadable media.

With reference to FIG. 11, an exemplary system for implementing anembodiment of the invention includes a computing device, such ascomputing device 100. In its most basic configuration, computing device100 typically includes at least one processing unit 103 and memory 104.

Depending on the exact configuration and type of computing device,memory 104 may be volatile (such as random-access memory (RAM)),non-volatile (such as read-only memory (ROM), flash memory, etc.) orsome combination of the two. This most basic configuration isillustrated in FIG. 11 by dashed line 106.

Additionally, device 100 may have additional features/functionality. Forexample, device 100 may also include additional storage (removableand/or non-removable) including, but not limited to, magnetic or opticaldisks or tape. Such additional storage is illustrated in FIG. 11 byremovable storage 108 and non-removable storage 110. Computer storagemedia includes volatile and nonvolatile, removable and non-removablemedia implemented in any method or technology for storage of informationsuch as computer-readable instructions, data structures, program modulesor other data. Memory 104, removable storage 108 and non-removablestorage 110 are all examples of computer storage media. Computer storagemedia includes, but is not limited to, RAM, ROM, EEPROM, flash memory orother memory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed bydevice 100. Any such computer storage media may be part of device 100.

Device 100 may also contain communications connection(s) 112 that allowthe device to communicate with other devices. Communicationsconnection(s) 112 is an example of communication media. Communicationmedia typically embodies computer-readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, radio-frequency (RF), infrared and other wireless media. Theterm computer-readable media as used herein includes both storage mediaand communication media.

Device 100 may also have input device(s) 114 such as keyboard, mouse,pen, voice-input device, touch-input device, etc. Output device(s) 116such as a display, speakers, printer, etc. may also be included.

An embodiment of the present invention pertains to interferometric fiberoptic gyroscopes (IFOGs), and particularly to depolarized IFOGs usingnon-polarization-maintaining SM fiber in the sensing coil. Moreparticularly, an embodiment of the present invention pertains to thereduction of bias instability originated from polarization errors.Embodiments include both analytical/computational and empirical methodsof testing and designing such gyroscopes.

The optical circuit 1 of a depolarized gyroscope is shown in FIG. 1.Depending on the embodiment, the circuit 1 may be computationallymodeled using computer-executable instructions or may be a tangibledevice. A broadband light source 11 generates a lightwave {right arrowover (E)}₀ that is composed of many waves with different wavelengths andpolarization states at the splice 30. In the general case, the lightemitted by broadband light source 11 may be described as a summation ofmany optical waves, i.e. {right arrow over (E)}₀=Σ{right arrow over(E)}₀(λ,|p

), where λ is wavelength (the spectrum of which ranges from λ₁ to λ₁)and |p

is a vector on the Poincaré sphere representing a particularpolarization state. The phases of these waves with different wavelengthsand polarization states may be uncorrelated. For a perfect unpolarizedlight source, the light intensity associated with all orthogonalpolarization states will be identical. When the unpolarized light passesthrough an optical component with polarization-dependent loss (PDL), thelightwaves of orthogonal polarization states experience different lossesand acquire different intensity at the output of the component.

A strict theoretical description of a broadband light source may requiretaking into account all the wavelength components and polarizationstates. However, for simplicity of the following discussion and withoutloss of generality, it may be assumed that the light source onlycontains two orthogonal polarization states for each wavelengthcomponent arriving at splice 30 in FIG. 1. The electric field of thebroadband light source 11 can thus be expressed by a Jones vector

$\begin{matrix}{{E_{0}(\lambda)} = {\begin{pmatrix}E_{0x} & (\lambda) \\E_{0y} & (\lambda)\end{pmatrix} = {{E_{0x}(\lambda)}{\begin{pmatrix}1 \\{t_{11y}(\lambda)}\end{pmatrix}.}}}} & (1)\end{matrix}$Here we may assume the PDL experienced by the light source before point30 is along the Y axis (i.e., the polarization state that experiencesmore loss is along the axis orthogonal to the plane of paper in FIG. 1).The amplitude transmission coefficient t_(11y) (λ) quantifies the degreeof polarization or PDL of light emitted by the light source 11 whenarriving at splice 30.

The light fields reaching the photodetector 14 can be modeled using aJones-matrix description of the gyro optical circuit 1. Element 12 canbe any kind of directional coupler. Here, a 2×2 coupler is used as anexample, which may include polarization maintaining (PM) fiber leads.There is normally a rotation angle α between the input light PDL axisand the PM fiber pass axis. The Jones matrix of such an angle rotationat splice 30 can be expressed as

$\begin{matrix}{R_{30} = {\begin{pmatrix}{\cos\mspace{11mu}\alpha} & {\sin\mspace{11mu}\alpha} \\{{- \sin}\mspace{11mu}\alpha} & {\cos\mspace{11mu}\alpha}\end{pmatrix}.}} & (2)\end{matrix}$

If light emitted by the light source 11 experiences wavelength dependentPDL, the rotation angle α(λ) may be a function of wavelength λ. Forsimplicity, we implicitly assume the wavelength dependence of everyparameter and omit the λ in the following expressions.

The propagation of light through the directional coupler 12, includingits fiber leads, can be described by a Jones matrix H_(in). This matrixtakes into account the polarization dependent splitting ratio, PDL,cross-couplings and phase delays incurred by the coupler 12 as lightpropagates from splice 30 to splice 31.

Element 15 may be a piece of PM fiber that connects the coupler 12 andan integrated optical circuit (IOC) 16. Element 15 has a Jones matrix of

$\begin{matrix}{{D_{15} = \begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{{- {\mathbb{i}}}\; d_{15}}\end{pmatrix}},} & (3)\end{matrix}$where d₁₅ is the relative phase delay of the slow axis with respect tothe fast axis of the PM fiber 15. The IOC pigtails 33, 36 and 38 may bePM fibers. They can similarly be described by delay matrices

$\begin{matrix}{{D_{j} = \begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{{- {\mathbb{i}}}\; d_{j}}\end{pmatrix}},\mspace{14mu}{j = 33},36,38.} & (4)\end{matrix}$

The splices, 31 and 32, between PM fibers, and the connection points,34, 35 and 37, between IOC pigtail fibers and the IOC waveguides, arenot perfect. They introduce small polarization cross-couplings, whichcan be described by Jones matrices

$\begin{matrix}{{K_{j} = \begin{pmatrix}1 & k_{j} \\{- k_{j}} & 1\end{pmatrix}},\mspace{14mu}{j = 31},32,34,35,37.} & (5)\end{matrix}$

For light propagation from splice 30 to splice 34, the total Jonesmatrix G is

$\begin{matrix}{G = {\begin{pmatrix}G_{11} & G_{12} \\G_{21} & G_{22}\end{pmatrix} = {K_{34}D_{33}K_{32}D_{15}K_{31}H_{in}{R_{30}.}}}} & (6)\end{matrix}$The electric field at the input point 34 of the IOC waveguide istherefore

$\begin{matrix}{\begin{pmatrix}A_{x} \\A_{y}\end{pmatrix} = {{G\begin{pmatrix}E_{0x} \\E_{0y}\end{pmatrix}} = \begin{pmatrix}{G_{11}E_{0x}} & + & {G_{12}E_{0y}} \\{G_{21}E_{0x}} & + & {G_{22}E_{0y}}\end{pmatrix}}} & (7)\end{matrix}$

Next, a Jones matrix

$\quad\begin{pmatrix}M & N \\L & J\end{pmatrix}$is used to represent the transmission of light through the sensing loop10 in the clockwise direction, without taking into account the IOC phasemodulation and its polarization extinction ratio ε. For the depolarizedgyroscope shown in FIG. 1, the clockwise transmission matrix is

$\begin{matrix}{\quad{\begin{pmatrix}M & N \\L & J\end{pmatrix} = {D_{41}K_{37}D_{38}K_{25}D_{26}R_{27}D_{28}R_{29}C_{10}R_{24}D_{23}R_{22}D_{21}K_{20}D_{36}K_{35}{D_{40}.}}}} & (8)\end{matrix}$Here, D₄₀ and D₄₁ are delay matrices for the IOC waveguide 40 and 41.Delay matrices D₂₁, D₂₃, D₂₆, D₂₈ are for segments of PM fibers thatconstitute the depolarizer 200, which contains an upper section 210 anda lower section 220. The imperfect splices 20 and 25 with small anglemisalignments are represented by matrices K₂₀ and K₂₅. The two 45° anglesplices 22 and 27 between the two pair of depolarizer PM fiber segments21, 23 and 26, 28 are represented by matrices R₂₂ and R₂₇. Once can alsoinclude rotation matrices R₂₄ and R₂₉ for the splices 24 and 29 betweenthe depolarizer sections 210, 220 and the fiber leads 101, 102 of thesingle-mode sensing coil 10.

The Jones matrix for propagation of light in the counter-clockwisedirection is the transpose of the clockwise counterpart, i.e.

$\quad{\begin{pmatrix}M & L \\N & J\end{pmatrix}.}$Again, the IOC phase modulation and polarization property are not takeninto account in this matrix.

The integrated optical circuit (IOC) 16 may be composed of a polarizingelement 18 (alternatively, the polarizing element itself could be awaveguide), a beam splitter/combiner 17 and two or more electrodes 19for phase-modulation of the light beams passing through the IOCwaveguides 40 and 41. This phase modulation function of IOC is used forgyro bias modulation, which typically changes the phase of lightpolarized along the pass axis (the horizontal x-axis is assumed here)with a period equal to loop transit time τ. If a clockwise propagatinglightwave experiences a modulation phase φ_(U)(t) at the upperwaveguide, it experiences a different modulation phase φ_(U)(t+τ) at thelower waveguide. During the same time, the counter-clockwise wave canexperience a modulation phase of φ_(L)(t) at the lower waveguide andφ_(L)(t+τ) at the upper waveguide. For the situation of push-pullsquare-wave modulation, one can simply assume φ_(U)(t)=φ_(L)(t+τ)=φ_(B)and φ_(L)(t)=φ_(U)(t+τ)=−φ_(B) without loss of generality. In this case,the matrix that takes into account the IOC bias modulation andpolarization extinction ratio for clockwise light propagation is

$\begin{matrix}{\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}{\quad{{\begin{pmatrix}M & N \\L & J\end{pmatrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}},}}} & (9)\end{matrix}$and that for the counter clockwise propagation is

$\begin{matrix}{\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}\begin{pmatrix}M & L \\N & J\end{pmatrix}{\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}.}} & (10)\end{matrix}$

The combined electric fields of CW and CCW light at IOC output 34 aretherefore determined by the following equation:

$\begin{matrix}{\begin{pmatrix}B_{x} \\B_{y}\end{pmatrix} = {{\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}\begin{pmatrix}M & N \\L & J\end{pmatrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}\begin{pmatrix}A_{x} \\A_{y}\end{pmatrix}} + {\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}\begin{pmatrix}M & L \\N & J\end{pmatrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}\phi}_{B}} & 0 \\0 & ɛ\end{pmatrix}\begin{pmatrix}A_{x} \\A_{y}\end{pmatrix}}}} & (11)\end{matrix}$

Taking into account the Sagnac phase 2φ_(R) induced by the rotationsensing and expanding the above equation, yields:

$\begin{matrix}{B_{x} = {{{A_{x}( {{\mathbb{e}}^{{i\;\phi_{R}} + {i\; 2\phi_{B}}} + {\mathbb{e}}^{{{- i}\;\phi_{R}} - {i\; 2\phi_{B}}}} )}M} + {ɛ\;{A_{y}( {{{\mathbb{e}}^{{i\;\phi_{R}} + {i\;\phi_{B}}}N} + {{\mathbb{e}}^{{{- i}\;\phi_{R}} - {i\;\phi_{B}}}L}} )}}}} & (12) \\{B_{y} = {{ɛ\;{A_{x}( {{{\mathbb{e}}^{{i\;\phi_{R}} + {i\;\phi_{B}}}L} + {{\mathbb{e}}^{{{- i}\;\phi_{R}} - {i\;\phi_{B}}}N}} )}} + {ɛ^{2}{A_{y}( {{\mathbb{e}}^{i\;\phi_{R}} + {\mathbb{e}}^{{- i}\;\phi_{R}}} )}{J.}}}} & (13)\end{matrix}$

To obtain the electric field at the photodetector 14, matrix F is usedto describe the optic path from point 34 to the photodetector 14. It is:

$\begin{matrix}{F = {\begin{pmatrix}F_{11} & F_{12} \\F_{21} & F_{22}\end{pmatrix} = {T_{14}R_{13}H_{out}K_{31}^{T}D_{15}K_{32}^{T}D_{33}{K_{34}^{T}.}}}} & (14)\end{matrix}$Here K₃₂ ^(T), K₃₃ ^(T), K₃₄ ^(T) are the transposes of matrices K₃₂,K₃₃, K₃₄ respectively H_(out) is the coupler matrix for lighttransmission from splice 31 to photodetector 14. Matrix

$\begin{matrix}{R_{13} = \begin{pmatrix}{\cos\;\sigma} & {{- \sin}\;\sigma} \\{\sin\;\sigma} & {\cos\;\sigma}\end{pmatrix}} & (15)\end{matrix}$takes into account the mismatch angle of the PM fiber axis with respectto the axis of photodetector polarization-dependent responsivity, whichis accounted for by matrix

$\begin{matrix}{T_{14} = {\begin{pmatrix}1 \\t_{14_{y}}\end{pmatrix}.}} & (16)\end{matrix}$

With the matrix F, the electric field at the photodetector 14 can beexpressed as

$\begin{matrix}{\begin{pmatrix}E_{x} \\E_{y}\end{pmatrix} = {{F\begin{pmatrix}B_{x} \\B_{y}\end{pmatrix}} = \begin{pmatrix}{{F_{11}B_{x}} + {F_{12}B_{y}}} \\{{F_{21}B_{x}} + {F_{22}B_{y}}}\end{pmatrix}}} & (17)\end{matrix}$

The power of the horizontally polarized light at the photodetector 14 is

$\begin{matrix}{I_{x} = {{E_{x}E_{x}^{*}} = {{( {{F_{11}B_{x}} + {F_{12}B_{y}}} )( {{F_{11}B_{x}} + {F_{12}B_{y}}} )^{*}} = {{{F_{11}}^{2}B_{x}B_{x}^{*}} + {{F_{12}}^{2}B_{y}B_{y}^{*}} + {F_{11}F_{12}^{*}B_{x}B_{y}^{*}} + {F_{12}F_{11}^{*}B_{y}B_{x}^{*}}}}}} & (18)\end{matrix}$

Substituting (12) and (13) into (18) yields:

$\begin{matrix}\begin{matrix}{{I_{x}( \phi_{B} )} = {{{F_{11}}^{2}\begin{bmatrix}{{2{{A_{x}M}}^{2}( {1 + {\cos( {{2\phi_{R}} + {4\phi_{B}}} )}} )} +} \\{{2ɛ{{A_{x}A_{y}^{*}}}\begin{pmatrix}{{{MN}^{*}}( {{\cos( {\phi_{M - N} + \phi_{{Ax} - {Ay}} + \phi_{B}} )} +} } \\{ {\cos( {\phi_{M - N} + \phi_{{Ax} - {Ay}} - {2\phi_{R}} - {3\phi_{B}}} )} ) +} \\\begin{matrix}{{{ML}^{*}}( {{\cos( {\phi_{M - L} + \phi_{{Ax} - {Ay}} - \phi_{B}} )} +} } \\ {\cos( {\phi_{M - L} + \phi_{{Ax} - {Ay}} + {2\phi_{R}} - {3\phi_{B}}} )} )\end{matrix}\end{pmatrix}} +} \\{ɛ^{2}{A_{y}}^{2}( {{N}^{2} + {L}^{2} + {2{{NL}^{*}}{\cos( {\phi_{N - L} + {2\phi_{R}} + {2\phi_{B}}} )}}} )}\end{bmatrix}} + {{F_{12}}^{2}\begin{bmatrix}{{ɛ^{2}{A_{x}}^{2}( {{N}^{2} + {L}^{2} + {2{{NL}^{*}}{\cos( {\phi_{N - L} - {2\phi_{R}} - {2\phi_{B}}} )}}} )} +} \\{2ɛ^{3}{{A_{x}A_{y}^{*}}}\begin{pmatrix}\begin{matrix}( {{\cos( {\phi_{L - J} + \phi_{{Ax} - {Ay}} + \phi_{B}} )} +}  \\{{ {\cos( {\phi_{L - J} + \phi_{{Ax} - {Ay}} + {2\phi_{R}} + \phi_{B}} )} ){{LJ}^{*}}} +}\end{matrix} \\\begin{matrix}( {{\cos( {\phi_{N - J} + \phi_{{Ax} - {Ay}} - \phi_{B}} )} +}  \\{ {\cos( {\phi_{N - J} + \phi_{{Ax} - {Ay}} - {2\phi_{R}} - \phi_{B}} )} ){{NJ}^{*}}}\end{matrix}\end{pmatrix}}\end{bmatrix}} + {{F_{11}F_{12}^{*}}}}} \\\lbrack \begin{matrix}{{2ɛ{A_{x}}^{2}\begin{pmatrix}\begin{matrix}{{{MN}^{*}}( {{\cos( {\phi_{{F\; 11} - 12} + \phi_{M - N} - \phi_{B}} )} +} } \\{ {\cos( {\phi_{{F\; 11} - 12} + \phi_{M - N} + {2\phi_{R}} - {3\phi_{B}}} )} ) +}\end{matrix} \\\begin{matrix}{{{ML}^{*}}( {\cos( {\phi_{{F\; 11} - 12} + \phi_{M - L} + \phi_{B}} )} } \\ {{+ \cos}( {\phi_{\;_{{F\; 11} - 12}} + \phi_{M - L} - {2\phi_{R}} - {3\phi_{B}}} )} )\end{matrix}\end{pmatrix}} +} \\{{2ɛ^{2}{{A_{y}A_{x}^{*}}}\begin{pmatrix}\begin{matrix}{{{N}^{2}{\cos( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}} +} \\{{{L}^{2}{\cos( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} + {2\phi_{R}} + {2\phi_{B}}} )}} +}\end{matrix} \\\begin{matrix}{{{NL}^{*}}( {{\cos( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} - \phi_{N - L}} )} +} } \\ {\cos( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} + \phi_{N - L}} )} )\end{matrix}\end{pmatrix}} +} \\{{2ɛ^{2}{{A_{x}A_{y}^{*}}}{{MJ}^{*}}\begin{pmatrix}\begin{matrix}{{\cos( {\phi_{{F\; 11} - 12} + \phi_{M - J} + \phi_{{Ax} - {Ay}} + {2\phi_{B}}} )} +} \\{{\cos( {\phi_{{F\; 11} - 12} + \phi_{M - J} + \phi_{{Ax} - {Ay}} - {2\phi_{B}}} )} +}\end{matrix} \\\begin{matrix}{{\cos( {\phi_{{F\; 11} - 12} + \phi_{M - J} + \phi_{{Ax} - {Ay}} + {2\phi_{R}} + {2\phi_{B}}} )} +} \\{\cos( {\phi_{{F\; 11} - 12} + \phi_{M - J} + \phi_{{Ax} - {Ay}} - {2\phi_{R}} - {2\phi_{B}}} )}\end{matrix}\end{pmatrix}} +} \\{2ɛ^{3}{A_{y}}^{2}\begin{pmatrix}\begin{matrix}{{{NJ}^{*}}( {{\cos( {\phi_{{F\; 11} - 12} + \phi_{N - J} + \phi_{B}} )} +} } \\{ {\cos( {\phi_{{F\; 11} - 12} + \phi_{N - J} + {2\phi_{R}} + \phi_{B}} )} ) +}\end{matrix} \\\begin{matrix}{{{LJ}^{*}}( {{\cos( {\phi_{{F\; 11} - 12} + \phi_{L - J} - \phi_{B}} )} +} } \\ {\cos( {\phi_{\;_{{F\; 11} - 12}} + \phi_{L - J} - {2\phi_{R}} - \phi_{B}} )} )\end{matrix}\end{pmatrix}}\end{matrix} \rbrack\end{matrix} & (19)\end{matrix}$

In equation 19, φ_(M-N), φ_(M-L), φ_(N-L), φ_(L-J) and φ_(Ax-Ay) expressthe phase differences of the complex numbers M and N; M and L; N and L;L and J; A_(x) and A_(y). Similarly, term φ_(F11-12) represents thephase differences of complex numbers F₁₁ and F₁₂.

Demodulation of the above push-pull square-wave bias modulated signal isaccomplished by calculating the differences of the light intensity inthe neighboring time slots of loop transit time τ. Since the φ_(B) flipsthe sign in the neighboring period, τ, the demodulated signal iscalculated by

$\begin{matrix}{I_{x\_ dm} = {{I_{x}( {- \phi_{B}} )} - {I_{x}( \phi_{B} )}}} & (20)\end{matrix}$

For the primary signal (i.e., the first term in Eq. (19)), thedemodulated signal is

$\begin{matrix}{I_{{main\_ x}{\_ dm}} = {{2{F_{11}}^{2}{{A_{x}M}}^{2}( {{\cos( {{2\phi_{R}} - {4\phi_{B}}} )} - {\cos( {{2\phi_{R}} + {4\phi_{B}}} )}} )} = {4{F_{11}}^{2}{{A_{x}M}}^{2}{\sin( {2\phi_{R}} )}\sin\;( {4\phi_{B}} )}}} & (21)\end{matrix}$

Demodulation of other terms in (19) yields two major types ofpolarization errors. One type is called amplitude type error, which isproportional to IOC polarization extinction ratio ε. The other type iscalled intensity type error, which is proportional to ε². Higher orderterms are extremely small, and therefore are neglected for purposes ofsimplicity in the following discussion. Performing demodulationcalculation similar to those in Equations (20) and (21), theamplitude-type polarization error light intensity along the x-axis is:

$\begin{matrix}{I_{{amp\_ x}{\_ dm}} = {\quad{{4 ɛ{F_{11}}^{2}{{A_{x} A_{y}^{*}}}\begin{pmatrix}{{{{MN}^{*}}\begin{pmatrix}{{\sin( {\phi_{M - N} + \phi_{{Ax} - {Ay}}} )}{\sin( \phi_{B} )}} \\{{- {\sin( {\phi_{M - N} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}} +} \\{{{ML}^{*}}\begin{pmatrix}{{- {\sin( {\phi_{M - L} + \phi_{{Ax} - {Ay}}} )}}{\sin( \phi_{B} )}} \\{{+ {\sin( {\phi_{M - L} + \phi_{{Ax} - {Ay}} + {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}}\end{pmatrix}} + {4ɛ{{F_{11}F_{12}^{*}}}^{2}{A_{x}}^{2}{\begin{pmatrix}{{{{MN}^{*}}\begin{pmatrix}{{- {\sin( {\phi_{{F\; 11} - 12} + \phi_{M - N}} )}}{\sin( \phi_{B} )}} \\{{+ {\sin( {\phi_{{F\; 11} - 12} + \phi_{M - N} + {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}} +} \\{{{ML}^{*}}\begin{pmatrix}{{\sin( {\phi_{{F\; 11} - 12} + \phi_{M - L}} )}{\sin( \phi_{B} )}} \\{{- {\sin( {\phi_{{F\; 11} - 12} + \phi_{M - L} - {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}}\end{pmatrix}.}}}}} & (22)\end{matrix}$

The intensity-type polarization error light intensity along the x-axisis

$\begin{matrix}{ {{{{{{I_{{{int}{\_ x}}{\_ dm}} = {{4ɛ^{2}{F_{11}}^{2}{A_{y}}^{2}{{NL}^{*}}{\sin( {\phi_{N - L} + {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} - {4ɛ^{2}{F_{12}}^{2}{A_{x}}^{2}{{NL}^{*}}{\sin( {\phi_{N - L} - {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} +}}\;\quad}4 ɛ^{2}{{F_{11} F_{12}^{*}}}{{A_{y} A_{x}^{*}}}( \begin{matrix}{{- {N}^{2}}{\sin( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} \\{{+ {L}^{2}}{\sin( {\phi_{{F\; 11} - 12} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}}\end{matrix} )} + {{\quad\quad}8ɛ^{2}{{F_{11}F_{12}^{*}}}{{A_{x}A_{y}^{*}}}{{MJ}^{*}}{\sin( {2\phi_{R}} )}{\cos( {\phi_{{F\; 11} - 12} +}\quad }\phi_{M - J}} +}\quad}\phi_{{Ax} - {Ay}}} ){{\sin( {2\phi_{B}} )}.}} & (23)\end{matrix}$

In a similar manner, we can obtain the demodulated main signal along they-axis

$\begin{matrix}{I_{{main\_ y}{\_ dm}} = {4{F_{21}}^{2}{{A_{x}M}}^{2}{\sin( {2\phi_{R}} )}{{\sin( {4\phi_{B}} )}.}}} & (24)\end{matrix}$

The amplitude-type polarization error intensity along the y-axis is

$\begin{matrix}{I_{{amp\_ y}{\_ dm}} = {\quad{{4 ɛ{F_{21}}^{2}{{A_{x} A_{y}^{*}}}\begin{pmatrix}{{{{MN}^{*}}\begin{pmatrix}{{\sin( {\phi_{M - N} + \phi_{{Ax} - {Ay}}} )}{\sin( \phi_{B} )}} \\{{- {\sin( {\phi_{M - N} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}} +} \\{{{ML}^{*}}\begin{pmatrix}{{- {\sin( {\phi_{M - L} + \phi_{{Ax} - {Ay}}} )}}{\sin( \phi_{B} )}} \\{{+ {\sin( {\phi_{M - L} + \phi_{{Ax} - {Ay}} + {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}}\end{pmatrix}} + {4ɛ{{F_{21}F_{22}^{*}}}{A_{x}}^{2}\begin{pmatrix}{{{{MN}^{*}}\begin{pmatrix}{{- {\sin( {\phi_{{F\; 21} - 22} + \phi_{M - N}} )}}{\sin( \phi_{B} )}} \\{{+ {\sin( {\phi_{{F\; 21} - 22} + \phi_{M - N} + {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}} +} \\{{{ML}^{*}}\begin{pmatrix}{{\sin( {\phi_{{F\; 21} - 22} + \phi_{M - L}} )}{\sin( \phi_{B} )}} \\{{- {\sin( {\phi_{{F\; 21} - 22} + \phi_{M - L} - {2\phi_{R}}} )}}{\sin( {3\phi_{B}} )}}\end{pmatrix}}\end{pmatrix}}}}} & (25)\end{matrix}$

The intensity-type polarization error along the y-axis is

$\begin{matrix}{ {{{{{{I_{{{int}{\_ y}}{\_ dm}} = {{4ɛ^{2}{F_{21}}^{2}{A_{y}}^{2}{{NL}^{*}}{\sin( {\phi_{N - L} + {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} - {4ɛ^{2}{F_{22}}^{2}{A_{x}}^{2}{{NL}^{*}}{\sin( {\phi_{N - L} - {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} +}}\;\quad}4 ɛ^{2}{{F_{21} F_{22}^{*}}}{{A_{y} A_{x}^{*}}}( \begin{matrix}{{- {N}^{2}}{\sin( {\phi_{{F\; 21} - 22} + \phi_{{Ax} - {Ay}} - {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}} \\{{+ {L}^{2}}{\sin( {\phi_{{F\; 21} - 22} + \phi_{{Ax} - {Ay}} + {2\phi_{R}}} )}{\sin( {2\phi_{B}} )}}\end{matrix} )} + {{\quad\quad}8ɛ^{2}{{F_{21}F_{22}^{*}}}{{A_{x}A_{y}^{*}}}{{MJ}^{*}}{\sin( {2\phi_{R}} )}{\cos( {\phi_{{F\; 21} - 22} +}\quad }\phi_{M - J}} +}\quad}\phi_{{Ax} - {Ay}}} ){\sin( {2\phi_{B}} )}} & (26)\end{matrix}$

The electric fields, A_(x) and A_(y), in expressions from (21) to (26)can be replaced by the incoming broadband light source fields, E_(0x)and E_(0y). After applying Eq. (7), the intensity |A_(x)|², |A_(y)|² andA_(x)A*_(y) can be expanded into the following form:

$\begin{matrix}{{A_{x}}^{2} = {{( {{G_{11}E_{0x}} + {G_{12}E_{0y}}} )( {{G_{11}E_{0x}} + {G_{12}E_{0y}}} )^{*}} = {{{G_{11}}^{2}{E_{0x}}^{2}} + {{G_{12}}^{2}{E_{0y}}^{2}}}}} & (27) \\{{A_{y}}^{2} = {{( {{G_{21}E_{0x}} + {G_{22}E_{0y}}} )( {{G_{21}E_{0x}} + {G_{22}E_{0y}}} )^{*}} = {{{G_{21}}^{2}{E_{0x}}^{2}} + {{G_{22}}^{2}{E_{0y}}^{2}}}}} & (28)\end{matrix}$

In the above equations, the terms that have phase difference of E_(0x)and E_(0y) components can be ignored because they are uncorrelated andaverage to zero over time.

Summing the intensities along both x- and y-axes, and using equations,E_(0x)(λ)=√{square root over (S(λ))}, E_(0y)(λ)=t_(11y)√{square rootover (S(λ))} implied in Eq. (1) for light with intensity S(λ) atwavelength λ, the main signal light intensity becomes

$\begin{matrix}{{I_{main\_ dm}(\lambda)} = {4{S(\lambda)}( {{F_{11}}^{2} + {F_{21}}^{2}} )( {{G_{11}}^{2} + {{G_{12}}^{2}{t_{11y}}^{2}}} ){M}^{2}{\sin( {2\phi_{R}} )}{{\sin( {4\phi_{B}} )}.}}} & (30)\end{matrix}$

The total main signal intensity is obtained through wavelengthintegration over the entire light source spectrum. From (30), the scalefactor to convert light intensity to rate in unit of deg/hr is obtained,

$\begin{matrix}{{SF} = {\frac{1}{\begin{matrix}{4{\int_{\lambda_{1}}^{\lambda_{2}}{{S(\lambda)}( {{F_{11}}^{2} + {F_{21}}^{2}} )( {{G_{11}}^{2} +} }}} \\{ {{G_{12}}^{2}{t_{11y}}^{2}} ){M}^{2}{\sin( {4\phi_{B}} )}\ {\mathbb{d}\lambda}}\end{matrix}} \cdot \frac{\overset{\_}{\lambda}c}{2\pi\; L_{coil}D_{coil}} \cdot \frac{180}{\pi} \cdot 3600.}} & (31)\end{matrix}$where L_(coil) and D_(coil) are the sensing coil 10 fiber length andaverage coil diameter, and λ is the light source mean wavelength. Thescale factor is used to convert spurious light intensities to bias errorwith unit of deg/hr.

The total amplitude type error, taking into account both x- and y-axes,is

$\begin{matrix}{{I_{amp\_ dm}(\lambda)} = {4ɛ\;{S(\lambda)}( {{F_{11}}^{2} + {F_{21}}^{2}} ){{G_{11}G_{21}^{*}}}( {{{{MN}^{*}}{\sin( {\phi_{M - N} + \phi_{{G\; 1\; 1} - 21}} )}} - {{{ML}^{*}}{\sin( {\phi_{M - L} + \phi_{{G\; 1\; 1} - 21}} )}}} ){\quad{( {{\sin( \phi_{B} )} - {\sin( {3\phi_{B}} )}} ) + {4ɛ\;{S(\lambda)}( {{F_{11}}^{2} + {F_{21}}^{2}} ){{G_{12} G_{22}^{*}}}{t_{11\; y}}^{2}( {{{{MN}^{*}}\sin\;( {\phi_{M - N} + \phi_{{G\; 1\; 2} - 22}} )} - { \quad{{{ML}^{*}}{\sin( {\phi_{M - L} + \phi_{{G\; 1\; 2} - 22}} )}} )( {\sin( \phi_{B} )} )} - ( {\sin( {3\phi_{B}} )} ) - {4ɛ\;{S(\lambda)}{{F_{11}F_{12}^{*}}}( {{G_{11}}^{2} + {{G_{12}}^{2}{t_{11\; y}}^{2}}} )( {{{{MN}^{*} }{\sin( {\phi_{{F\; 11} - 12} + \phi_{M - N}} )}} - {{{ML}^{*}}{\sin( {\phi_{{F\; 11} - 12} + \phi_{M - L}} )}}} )( {{\sin( \phi_{B} )} - {\sin( {3\phi_{B}} )}} )} - {4ɛ\;{S(\lambda)}{{F_{21}F_{22}^{*}}}( {{G_{11}}^{2} + {{G_{12}}^{2}{t_{11\; y}}^{2}}} )( {{{{MN}^{*}}{\sin( {\phi_{{F\; 21} - 22} + \phi_{M - N}} )}} - {{{ML}^{*}}{\sin( {\phi_{{F\; 21} - 22} + \phi_{M - L}} )}}} )( {{\sin( \phi_{B} )} - {\sin( {3\phi_{B}} )}} )}} }}}}} & (32)\end{matrix}$

The total intensity-type error is

$\begin{matrix}{{I_{{int}{\_ dm}}(\lambda)} = {{\quad\quad}4ɛ^{2}\;{S(\lambda)}( {{F_{11}}^{2} + { \quad{F_{21}}^{2} )( {{G_{21}}^{2} + {{G_{22}}^{2}{t_{11y}}^{2}}} ){{NL}^{*}}{\sin( \phi_{N - L} )}{\sin( {2\phi_{B}} )}} - {4ɛ^{2}\;{S(\lambda)}( {{F_{12}}^{2} + {F_{22}}^{2}} )( {{G_{11}}^{2} +}\quad  \quad{{G_{12}}^{2}{t_{11y}}^{2}} ){{NL}^{*}}{\sin( \phi_{N - L} )}{\sin( {2\phi_{B}} )}} - {4ɛ^{2}\;{S(\lambda)}{{F_{11}F_{12}^{*}}}{{G_{11}G_{21}^{*}}}( {{N}^{2} - {L}^{2}} ){\sin( {\phi_{{G\; 11} - 21} + {\quad{{ \quad\phi_{{F\; 11} - 21} ){\sin( {2\phi_{B}} )}} - {4ɛ^{2}\;{S( {{\lambda{{F_{11}F_{12}^{*}}}{{G_{12}G_{22}^{*}}}{t_{11y}}^{2}( {{N}^{2} - {L}^{2}} ){\sin( {\phi_{{G\; 12} - 22} + \phi_{{F\; 11} - 21}} )}{\sin( {2\phi_{B}} )}} - {4ɛ^{2}\;{S(\lambda)}{{F_{21}F_{22}^{*}}}{{G_{11}G_{21}^{*}}}( {{N}^{2} - {{L}^{2}{\sin( {\phi_{{G\; 11} - 21} + \phi_{{F\; 21} - 22}} )}{\sin( {2\phi_{B}} )}} - {\quad{4ɛ^{2}\;{S(\lambda)}{{F_{21}F_{22}^{*}}}{{G_{12}G_{22}^{*}}}{t_{11y}}^{2}( {{N}^{2} - {L}^{2}} ){\sin( {\phi_{{G\; 12} - 22} + \phi_{{F\; 21} - 22}} )}{\sin( {2\phi_{B}} )}}}} }} }}}}} }}} }} & (33)\end{matrix}$

The total error rate originated from both amplitude- and intensity-typepolarization errors may thus be:

$\begin{matrix}{{Bias} = {{SF} \cdot {\int_{\lambda_{1}}^{\lambda_{2}}{( {{I_{amp\_ dm}(\lambda)} + {I_{int\_ dm}(\lambda)}} )\ {{\mathbb{d}\lambda}.}}}}} & (34)\end{matrix}$

Integration over wavelength from λ₁ to λ₂ covers the broadband lightsource wavelength range. Data characterizing the total error rate and/orother error information may be displayed to output device 116.

The approach described above predicts a bias offset when there arepolarization errors. Since the polarization errors are the result ofinterference of waves propagating along different respective opticalpaths, relative changes of optical paths will cause the correspondingpolarization error to fluctuate. The said “optical path” depends on thepolarization state of the light. For a PM fiber, there are twopolarization modes. One mode is polarized along the so-called “slowpolarization axis” in the fiber cross-section. It acquires more phasedelays than the other polarization mode which is polarized along theorthogonal “fast polarization axis”. The slow mode effectively travelsalong a longer optical path than the fast mode. To quantify the phasedifference associated with the fast and slow modes in a PM fiber, it isadvantageous to express the lengths of fiber in terms of “beat length”(L_(B)), such that L_(B)=λ/Δn, where λ is the light wavelength, and Δnis the difference in effective mode refractive index between the fastand the slow polarization modes. Typically, L_(B) ranges from 1 mm to 4mm. The phase difference between slow and fast light after propagatingover fiber length of L_(B) is 2π. Typically, the fiber beat length L_(B)increases slightly when temperature increases (i.e., the temperature isramped). Changing the PM fiber temperature is equivalent to changing thephase difference of the slow and fast light waves, and is one of theeffective ways to excite polarization error induced bias fluctuations.For example, if the PM fiber 15 is assumed to have a length of v=10000L_(B), and the beat length L_(B) of the PM fiber has a temperaturesensitivity of 500 ppm/° C., the relative phase change of the fast andslow light will undergo five cycles per ° C., i.e. 10π/° C. FIG. 2illustrates a plot of such relative bias versus temperature change for agyroscope with polarization errors using an approach according to anembodiment of the invention. Since absolute bias depends on specificgyroscope design, only relative bias amplitude values in arbitrarylinear unit are used throughout the document without lost ofsignificances. To perform a real temperature ramp measurement, the gyrois put in a temperature chamber under constant rotation rate, e.g. theearth rotation rate. The temperature is gradually increased while ratesare continuously read from the gyro. If the gyro under test hassubstantial polarization errors, the rates will exhibit sinusoidaloscillation patterns versus temperature. The temperature period of theoscillation is a good indicator of the phase delay involved in thepolarization error. Measuring bias fluctuation under temperature rampingof selected sections of the optical circuit is an effective way toidentify the cause of the polarization error and allow a testing user toreplace or otherwise modify inferior portions in the circuit with moreoptimally configured portions. Additionally, based on theabove-described analytical approach, an embodiment includes analyticallysimulating the bias fluctuations under temperature ramping. Thetemperature ramping test and methods according to embodiments of theinvention are powerful tools in gyroscope polarization errorinvestigations.

In the calculation method developed above, the wavelength dependentJones matrix of the single mode non-polarization-maintaining fiber coilis used. The matrix can actually be measured directly using apolarization-mode-dispersion (PMD) characterization instrument. The coilmatrix can also be modeled by treating the coil as a concatenation ofmany random birefringent elements. With the availability of the coilJones matrix, the coherence function and the wavelength dependentdifferential group delay (DGD) of the coil can be determined. FIG. 3 ashows two coil coherence functions. Coil-I is represented by a solidline and coil-II is represented by a dashed line. These coil coherencefunctions, or autocorrelation functions, characterize interferencefringe contrast as a function of delay length of broadband lightwavespassing through the coil. At delay length of 2 mm, coil-I exhibits muchsmaller fringe contrast (−30 dB) compared to coil-II (−18 dB). FIG. 3 bshows the wavelength dependent DGDs and the averaged DGDs of thecorresponding coil-I and coil-II. Coil-I has an average DGD value of0.85 ps, much smaller than that of coil-II (dashed line, averageDGD=1.81 ps). In other words, coil-I has smaller internal birefringencethan coil-II. In the design of depolarized gyroscopes, fabricating coilswith low birefringence is advantageous in lowering polarization errors.FIG. 4 shows a comparison of relative bias fluctuation amplitude undertemperature ramping for two gyroscopes having identical design, exceptfor their respective coils, coil-I and II. FIG. 4 illustrates that thegyroscope using coil-II (dashed lines with circle marks) exhibitssignificantly larger bias instability than that using coil-I (solidline).

The coupler 12 in the optical circuit also plays an important role incontrolling polarization errors. As mentioned earlier, the Jones matrixof the coupler shall take into account the polarization dependentsplitting ratio (PDSR) and PDL. For light propagating from point 30 topoint 31, the x- and y-polarized light have different amplitudetransmission coefficient s_(x) and s_(y). There are also cross-couplingsbetween x- and y-polarized light that can be quantified by parameterk₁₂. After taking into account the phase delays, d₁₂₁, d₁₂₂ of the inputand output pigtails 121 and 122 of the coupler, the Jones matrix of thecoupler 12 can be written as

$\begin{matrix}{H_{in} = {{\begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{- {id}_{122}}\end{pmatrix}\begin{pmatrix}1 & k_{12} \\{- k_{12}} & 1\end{pmatrix}\begin{pmatrix}s_{x} & 0 \\0 & s_{y}\end{pmatrix}\begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{- {id}_{121}}\end{pmatrix}} \approx {s_{x}\begin{pmatrix}1 & {k_{12}{\mathbb{e}}^{- {id}_{121}}} \\{{- k_{12}}{\mathbb{e}}^{- {id}_{122}}} & {( \frac{s_{y}}{s_{x}} ){\mathbb{e}}^{- {i{({d_{121} + d_{122}})}}}}\end{pmatrix}} \approx \begin{pmatrix}1 & {k_{12}{\mathbb{e}}^{- {id}_{121}}} \\{{- k_{12}}{\mathbb{e}}^{- {id}_{122}}} & {{\mathbb{e}}^{- {i{({d_{121} + d_{122}})}}}\sqrt{10^{L_{PDSR}/10}}}\end{pmatrix}}} & (35)\end{matrix}$In the above equation L_(PDSR) is the polarization splitting ratiodifference in decibel units. Other PDL are omitted for simplification.Since the common loss and phase experienced by both x- and y-polarizedlight are not important in the final result of the polarization errorcalculation, the factor of s_(x) in the expression of (35) may beomitted in the last step. Similarly, for propagation of light from point31 to photodetector splice 13, the coupler matrix can be written as

$\begin{matrix}{H_{out} = {{\begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{- {id}_{123}}\end{pmatrix}\begin{pmatrix}r_{x} & 0 \\0 & r_{y}\end{pmatrix}\begin{pmatrix}1 & {- k_{12}} \\k_{12} & 1\end{pmatrix}\begin{pmatrix}1 & 0 \\0 & {\mathbb{e}}^{- {id}_{122}}\end{pmatrix}} \approx \begin{pmatrix}\sqrt{10^{L_{PDSR}/10}} & {{- k_{12}}{\mathbb{e}}^{- {id}_{122}}} \\{k_{12}{\mathbb{e}}^{- {id}_{123}}} & {\mathbb{e}}^{- {i{({d_{122} + d_{123}})}}}\end{pmatrix}}} & (36)\end{matrix}$where d₁₂₃ is the phase delay caused by the coupler pigtail 123.Although small PDSR has been assumed in the approximations ofexpressions, it is not intended to limit the application of the methodto use more general forms of H_(in) and H_(out) matrices. As illustratedin FIG. 5, the relative bias fluctuation amplitude under temperatureramping is found to decrease monotonically as PDSR approaches zero. Itis therefore advantageous to use a coupler having small PDSR and PDL toreduce the bias instability.

Most of the spurious light producing polarization errors originates fromcross-couplings at fiber splices or in optical components. It istherefore advantageous to reduce these cross-couplings as much aspossible. FIG. 6 shows a dependence of relative bias error as a functionof the cross-coupling at splice 32 (i.e., k₃₂). It is obvious thatsmaller k₃₂ produces smaller bias. A method, according to an embodiment,to reduce cross-couplings at splices between two PM fibers is to use theidentical or similar fiber types. The distortions involved at the jointpoints are reduced when the two connecting fibers have the same modefield diameter, stress elements and refraction index.

In the calculation of polarization error, one can observe that theamplitude-type polarization error is proportional to the IOCpolarization extinction ratio (PER), ε, and the intensity type error isproportional to ε². As an example, FIG. 7 shows a calculation forpolarization error induced relative bias instabilities as a function ofIOC PER. Indeed, by selecting IOC with smaller PER, bias fluctuationamplitude may be reduced.

Design of depolarizer 200 and the PM fiber 15 are effective ways toreduce gyro bias error according to an embodiment. As illustrated in theexample of FIG. 1, the depolarizer 200 is composed of two sections, 210and 220. Section 210 is composed of two pieces of PM fiber, 21 and 23,with polarization axis oriented about 45° with each other at splice 22.Section 220 is composed of two pieces of PM fiber, 26 and 28, withpolarization axis oriented about 45° with each other at splice 27.Section 15 is a piece of PM fiber that generates delays for the slow andfast light before such light enters the IOC 16. The lengths of PM fibers15, 21, 23, 26 and 28 are designated in FIG. 1 as, respectively, v, z,x, y and w. Together with IOC pigtail and waveguide lengths, 33, 36, 38,40 and 41, determination of the lengths of PM fibers 15, 21, 23, 26 and28 are important in the design of depolarized gyroscopes from thestandpoint of controlling polarization error. These lengths determinethe relative time delays involved in the broadband light sourceinterference. Longer delays cause decrease of interference fringecontrast and less bias fluctuations under temperature changes. Bytailoring the lengths of these PM fibers, it is possible to control thepolarization error below a certain value. For example, FIG. 8 shows, inassociation with gyroscopes employing, respectively, coil-I and coil-IIdiscussed above, a dependence of polarization error induced relativebias fluctuation amplitude on the length of PM fiber 15 in unit of beatsfor a design of depolarizer 200 according to an embodiment. Selectinglength, v, far away from an error peaking region is an effective way toreduce polarization error induced bias instability.

In an embodiment, the length, w, is selected to be approximately twicethat of length, x (i.e., w=2x). In addition, the physical lengths of thedepolarizer section 210 and 220 are kept approximately the same (i.e.,z=x+y). Sections 210, 220 may be winded onto the coil 10 with thesymmetrical points, i.e. points with equal optical length measured fromsplitting point 17 in IOC, located close to each other to avoid thetemperature-fluctuation-induced non-reciprocal phase noise (i.e., theso-called Shupe effect induced noise). To minimize the overalldepolarizer length, the length, y, and IOC pigtail lengths, 33, 36 and37 may be selected to be as short as possible. In such an embodiment ofthe model, the optimal lengths of x and v are determined once the IOC,coupler and coil properties are known.

As an example of showing the method to optimize the depolarizer and PMfiber 15 design, the calculated relative bias fluctuation amplitudeunder temperature ramp is plotted as functions of x and v in the contourplots illustrated in FIG. 9 a and FIG. 9 b. FIG. 9 a is plotted for agyroscope using the coil-I associated with FIG. 3 that has relativelysmaller birefringence. FIG. 9 b is plotted for a gyroscope using thecoil-II associated with FIG. 3 that has relatively larger birefringence.In both figures, dark regions have larger bias errors than those in thelight regions. Same color codes are used for FIGS. 9 a and 9 b forcomparison. Several v-x regions exhibit low bias errors and may thus bepreferable. These regions are marked as A, B, C, D and E. They arelocated between five dark regions with large bias fluctuationamplitudes. From comparison of FIGS. 9 a and 9 b, it may be seen thatthe coil birefringence has significant impact on the gyroscope design.Larger coil birefringence causes minimization of the optimal-designregions and larger background bias errors in the relatively optimaldesign regions.

Still referring to FIGS. 9 a and 9 b, the four narrow dark regions,roughly along the southwest-northeast direction, are found to have v-xrelations close to:

$\begin{matrix}\begin{matrix}{v = {{4x} + y - d_{32 - 27}}} & {{{ridge}\mspace{14mu}{between}\mspace{14mu} A\mspace{14mu}{and}\mspace{14mu} B},} \\{v = {{3x} - d_{32 - 27}}} & {{{ridge}\mspace{14mu}{between}\mspace{14mu} B\mspace{14mu}{and}\mspace{14mu} C},} \\{v = {{2x} - d_{32 - 27}}} & {{{ridge}\mspace{14mu}{between}\mspace{14mu} C\mspace{14mu}{and}\mspace{14mu} D},} \\{v = {x - d_{32 - 27}}} & {{ridge}\mspace{14mu}{between}\mspace{14mu} D\mspace{14mu}{and}\mspace{14mu}{E.}}\end{matrix} & (37)\end{matrix}$where d₃₂₋₂₇=L_(IOC)+y+d₃₃+d₃₈ is the relative phase delay of the fastand slow light of the optical path between 32 and 27, which includes theIOC waveguide 41, pigtails 33 and 38, and the depolarizer PM fiber 26.These regions of high bias instability are associated with amplitudetype polarization errors originating from the interference of theIOC-passed (x-polarized) waves and the IOC-rejected (y-polarized) wavestraveling through the sensing loop 10. A preferable choice of design isoften situated in the region A because of its larger low error regionproviding greater margins against coil uncertainties. However, designsin region A may require longer lengths of depolarizer and PM fibers 15,which may be of concern for gyro packaging and control of Shupe effect.Regions B, C, D and E are therefore of interest in design choices forreducing the PM fiber lengths.

However, the depolarizer x length cannot be lowered below a certainvalue. The broader vertically oriented regions on the left sides ofFIGS. 9 a and 9 b can undermine bias performance if the length x is toosmall. This region is associated with polarization errors insensitive tothe PM fiber 15 length v. Such errors mostly originate from interferenceamong waves passing through the PM fiber 15 in the same polarizationaxis. As depolarizer length x increases, this type of error decreases.

As another illustration of an embodiment of a depolarized gyroscope, onecan optimize the lengths x and w as separate parameters, rather thanobserving the relation w=2x. In such an embodiment, the length y can beset to a fixed value, and w+y is set equal to the sum of x and z (i.e.,z=y+w−x). As such, the depolarizer sections 210 and 220 have the samelength. Next, the PM fiber 15 may be first set to a large value. Asdiscussed elsewhere herein, it is known that v-insensitive error can beminimized by setting the length v, to a very large value. The design maynow be reduced to the determination of the optimal lengths of x and w.Once optimal x and w is found to minimize the v insensitive polarizationerror, the PM fiber 15 length v can then be determined to reduce the vsensitive polarization errors. One will see below that w=2x used in anearlier-discussed embodiment is indeed one of the optimal selections ofthe w-x relationship for better bias stability.

FIG. 10 a and FIG. 10 b illustrate plots of relative bias fluctuationamplitude calculation results as functions of x and w using coil-I andcoil-II, respectively. There are multiple design regions providing lowbias instability. Among them, regions F, G, H, and J are often ofgreater interest because they offer relatively shorter depolarizer fiberlengths. Region F has an approximate relationship between w and x ofw=x/2 or x=2w. Region G has a relation of w=x, which means the twodepolarizer PM fiber lengths are close to identical. Region H has arelation of w=2x, which was selected in an above-described embodimentand used in generating FIGS. 9 a and 9 b. Regions H and F are of thesame type from the point of view of symmetry (i.e., one of the x or wlengths is twice of the other). Region J is a wider region and may havebetter bias stabilities than those of region F, G and H.

An embodiment includes using a temperature-ramping approach, forexample, to select optimized values of depolarizer fiber section lengthw and x substantially in the regions of F, G, H and J shown in FIGS. 10a and b, to minimize v-insensitive polarization errors while setting vat a substantially long length. In one exemplary embodiment, w issubstantially equal to 8000 L_(B), x is substantially equal to 4000L_(B), y is substantially equal to 500 L_(B), and z substantially equalto 4500 L_(B). Additionally, the v may be substantially longer than16500 L_(B) (region-A in FIGS. 9 a and 9 b), substantially equal to12500 L_(B) (region-B in FIGS. 9 a and 9 b), equal to 8000 L_(B)(region-C in FIGS. 9 a and 9 b), substantially equal to 4000 L_(B)(region-D in FIGS. 9 a and 9 b), or substantially less than 1000 L_(B)(region-E in FIGS. 9 a and 9 b).

The relative bias amplitude in these regions can be clearly seen byreferring back to FIG. 8, which is a plot of bias versus PM fiberlength, v, when the x=4000 L_(B), y=500 L_(B) and w=8000 L_(B) areselected. Again, the solid line is associated with the coil-I and thedashed line associated with the coil-II. As is illustrated, the designsusing larger v-value regions (A, B and C) around v>16500 L_(B), v=12500and 8000 L_(B) have better tolerance to fiber length uncertainties thanthose using smaller v-values regions (D and E) around v=4000 L_(B) andv<1000 L_(B).

While preferred embodiments of the invention have been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

1. A fiber optic gyroscope comprising: a broadband light source havingan output fiber end; a directional coupler having a first optical fiberport, a second optical fiber port and a third optical fiber port, thefirst optical fiber port coupled to the light source output fiber end;wherein light entering the first optical fiber port is directed to thesecond fiber optical port, and light entering the second optical fiberport is directed to the third optical fiber port; a photodetectorcoupled to the third optical fiber port of the directional coupler; apolarization maintaining (PM) delay management fiber of length v, havinga first end and a second end, wherein the first end of the polarizationmaintaining delay management fiber is coupled to the second opticalfiber port of the directional coupler; an integrated optical circuit(IOC) having an input end coupled to the second end of the polarizationmaintaining delay management fiber via a first polarization maintainingfiber pigtail of length d₁; a first output end coupled to a secondpolarization maintaining fiber pigtail with length of d₂; a secondoutput end coupled to a third polarization maintaining fiber pigtailwith length of d₃; and a splitter that directs light coupled into theinput end to the first output end and the second output end withsubstantially equal power; wherein the integrated optic circuit (IOC)comprises a polarization maintaining waveguide device having waveguidelength of L_(IOC) and comprise a polarizing element so that the totaleffective polarization extinction ratio (PER) of the integrated opticcircuit is ε; a single mode optical fiber loop having a first end and asecond end; and a depolarizer region coupled to said fiber loop, saiddepolarizer region including a first optical fiber section of length x,a second optical fiber section of length z, a third optical fibersection of length w and a fourth optical fiber section of length y, eachcomprising polarization maintaining fibers having a beat length ofL_(B); wherein the second optical fiber section is coupled to the firstoptical fiber section via a first splice, the first optical fibersection is coupled to the first end of the single mode optical fiberloop via a second splice, the fourth optical fiber section is coupled tothe third optical fiber section via a third splice, and the thirdoptical fiber section is coupled to the second end of the single modeoptical fiber loop via a fourth splice and wherein x+z is substantiallyequal to w+y; wherein the lengths of v, x, y, d₁ and d₃ are proportionedto avoid regions of high bias instability defined by the expressionsv=x−(L _(IOC) +y+d ₁ +d ₃),v=2x−(L _(IOC) +y+d ₁ +d ₃),v=3x−(L _(IOC) +y+d ₁ +d ₃), andv=4x+y−(L _(IOC) +y+d ₁ +d ₃).
 2. The fiber optic gyroscope of claim 1,wherein said first splice having an alignment substantially close to 45°between a major axis of polarization of said first fiber section and amajor axis of polarization of said second fiber section; and said thirdsplice having an alignment substantially close to 45° between a majoraxis of polarization of said third fiber section and a major axis ofpolarization of said fourth fiber section.
 3. The fiber optic gyroscopeof claim 1, wherein said second fiber section in the depolarizer regionis coupled to said second IOC fiber pigtail via a fifth splice; saidfourth fiber section in the depolarizer region is coupled to said thirdIOC fiber pigtail via a sixth splice; said fifth splice having analignment substantially close to 0° between a major axis of polarizationof said second fiber section and a major axis of polarization of saidsecond piece of IOC fiber pigtail; and said sixth splice having analignment substantially close to 0° between a major axis of polarizationof said fourth fiber section and a major axis of polarization of saidthird piece of IOC fiber pigtail.
 4. The fiber optic gyroscope of claim1, wherein a polarization axes of the first polarization maintainingfiber pigtail, the second polarization maintaining fiber pigtail and thethird polarization maintaining fiber pigtail are aligned substantiallywith a waveguide polarization axis of the integrated optic circuit. 5.The fiber optic gyroscope of claim 1, wherein w is substantially equalto n*x, and n is an integer.
 6. The fiber optic gyroscope of claim 5,wherein n=2.
 7. The fiber optic gyroscope of claim 1, wherein x issubstantially larger than 2000 L_(B).
 8. The fiber optical gyroscope ofclaim 1, wherein a differential group delay (DGD) of the single modeoptical fiber loop is substantially less than 1.81 ps.
 9. The fiberoptical gyroscope of claim 1, wherein the broadband light source issubstantially un-polarized at a point before coupling into thedirectional coupler.
 10. The fiber optical gyroscope of claim 1, whereinthe directional fiber coupler has a polarization dependent splittingratio less than −0.30 dB.
 11. The fiber optical gyroscope of claim 1,wherein polarization cross-coupling is less than −20 dB except at thefirst splice and the second splice.
 12. An optical circuit for a fiberoptic gyroscope having a broadband light source and a single modeoptical fiber loop, the circuit comprising: a polarization maintaining(PM) delay management fiber of length v, having a first end and a secondend, wherein the first end of the polarization maintaining delaymanagement fiber is coupled a directional coupler that receives lightfrom the broadband light source; an integrated optical circuit (IOC)having the integrated optical circuit coupled to the second end of thepolarization maintaining delay management fiber via a first polarizationmaintaining fiber pigtail of length d₁; the integrated optical circuitcoupled to a second polarization maintaining fiber pigtail with lengthof d₂; and the integrated optical circuit coupled to a thirdpolarization maintaining fiber pigtail with length of d₃; wherein theintegrated optic circuit (IOC) comprises a polarization maintainingwaveguide device having waveguide length of L_(IOC) and comprise apolarizing element so that the total effective polarization extinctionratio (PER) of the integrated optic circuit is ε; and a splitter thatsplits light received from the first polarization maintaining fiberpigtail into a first beam directed to the second polarizationmaintaining fiber pigtail and a second beam directed to the thirdpolarization maintaining fiber pigtail; and a depolarizer circuitcoupled to said fiber loop, said depolarizer circuit including a firstoptical fiber section of length x, a second optical fiber section oflength z, a third optical fiber section of length w and a fourth opticalfiber section of length y, each comprising polarization maintainingfibers having a beat length of L_(B); wherein the second optical fibersection is coupled to the first optical fiber section via a firstsplice, the first optical fiber section is coupled to a first end of thesingle mode optical fiber loop via a second splice, the fourth opticalfiber section is coupled to the third optical fiber section via a thirdsplice, and the third optical fiber section is coupled to a second endof the single mode optical fiber loop via a fourth splice; and whereinx+z is substantially equal to w+y; wherein the lengths of v, x, y, d₁and d₃ are proportioned to avoid regions of high bias instabilitydefined by the expressionsv=x−(L _(IOC) +y+d ₁ +d ₃),v=2x−(L _(IOC) +y+d ₁ +d ₃),v=3x−(L _(IOC) +y+d ₁ +d ₃), andv=4x+y−(L _(IOC) +y+d ₁ +d ₃).
 13. The optical circuit of claim 12,wherein said first splice having an alignment substantially close to 45°between a major axis of polarization of said first fiber section and amajor axis of polarization of said second fiber section; and said thirdsplice having an alignment substantially close to 45° between a majoraxis of polarization of said third fiber section and a major axis ofpolarization of said fourth fiber section.
 14. The optical circuit ofclaim 12, wherein said second fiber section in the depolarizer region iscoupled to said second IOC fiber pigtail via a fifth splice; said fourthfiber section in the depolarizer region is coupled to said third IOCfiber pigtail via a sixth splice; said fifth splice having an alignmentsubstantially close to 0° between a major axis of polarization of saidsecond fiber section and a major axis of polarization of said secondpiece of IOC fiber pigtail; and said sixth splice having an alignmentsubstantially close to 0° between a major axis of polarization of saidfourth fiber section and a major axis of polarization of said thirdpiece of IOC fiber pigtail.
 15. The optical circuit of claim 12, whereina polarization axes of the first polarization maintaining fiber pigtail,the second polarization maintaining fiber pigtail and the thirdpolarization maintaining fiber pigtail are aligned substantially with awaveguide polarization axis of the integrated optic circuit.
 16. Theoptical circuit of claim 12, wherein w is substantially equal to n*x,and n is an integer.
 17. The optical circuit of claim 12, wherein n=2.18. The optical circuit of claim 12, wherein x is substantially largerthan 2000 L_(B).
 19. The optical circuit of claim 12, wherein adifferential group delay (DGD) of the single mode optical fiber loop issubstantially less than 1.81 ps.
 20. The optical circuit of claim 12,wherein the directional fiber coupler has a polarization dependentsplitting ratio less than −0.30 dB.